wavenumber ranges. These include the asymmetric stretch in the vicinity of 1630-1670 cm-1, the symmetric stretch of CO2 (pertaining to carbonate) within the range of ~1295-1425 cm-1, the CO stretch between 990-1080 cm-1, the non-planar rocking motion bands at 830-880 cm-1, and the CO2 bending bands spanning 686-712 cm-1. Remarkably, these bands are consistent with reported features of alkali metal bicarbonates129, 131, such as NaHCO3. However, a distinctive feature emerges at 1281 cm-1, corresponding to another CO2 asymmetric stretch. This feature aligns with bidentate carbonate complexes wherein two oxygen atoms are coordinated with metal centers, as discussed previously129. Thus, the presence of this band, coupled with an additional feature found in the 1500-1550 cm-1 range, may suggest a different coordination of a carbonate ion with imidazolium cations, possibly orientated differently than the one with structure resembling monodentate alkali metal bicarbonates. These findings emphasize the need for a careful consideration of various BMMIM-CO3 complex structures for spectral interpretation in the context of CO2 reduction. We also identified a subtle difference in the spectra for imidazolium carbonate and imidazolium bicarbonate. For bicarbonate version, we observed a broad band at around 3100 cm-1, confidently assignable to the OH vibration of bicarbonate129. Formation of (bi)carbonate species in the electrochemical reduction of CO2 In the C=O region depicted in Figure 6.2, noticeable peaks at 1664 and 1636 cm-1 align well with the transmission spectra of [BMMIM2 CO3] and [BMMIM HCO3]. These features can be confidently assigned as vibrations corresponding to the asymmetric CO stretch, as discussed previously. The peak situated at 1445 cm-1 is unique for carbonate, and can be attributed to the CO symmetric stretch and is consistent with the presence of imidazolium carbonate complexes. Additionally, the observed bands at 1265 cm-1 and 1560 cm-1 can be attributed to another set of symmetric and asymmetric vibrations of (bi)carbonate species. Contrasting these result with
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